The present invention generally relates to seat belt retractors having energy-absorbing or dissipating mechanisms and more particularly to a multi-level load-limiting system, which includes a means for shifting the output characteristic of the retractor from one load level limit to another load level limit based on operational parameters.
Torsion bars have for decades been proposed for use in seat belt retractors as an energy absorbing/dissipating mechanism. As the torsion bar is twisted (absorbing or dissipating energy as the case may be) during a vehicle emergency, the retractor output, that is, the torque or force displacement (rotation) characteristic of the torsion bar quickly reaches a saturated region, which corresponds to its plastic range of operation. This somewhat constant characteristic provides a reaction torque at the retractor and provides a reaction force or load on the seat belt, which retards and controls the manner by which the seat belt protracts from the spool. One level of reaction forces may not be adequate to protect occupants of differing sizes. Consequently, it is desirable to provide a seat belt system with more than one load-limiting characteristic or one that can be changed or changes as dynamic conditions change.
The prior art illustrates seat belt retractors having two dissimilar and remotely located torsion bars to achieve a multi-level of load limiting, while other prior retractors use a single torsion bar that is sub-divided into two torsion bar portions to achieve multi-level load-limiting operation.
The present invention utilizes at least two energy dissipating/absorbing mechanisms to provide a system that can achieve performance standards in new United States Federal regulations (see FMVSS 209) as it relates to 5th, 50th and 95th percentile sized occupants. In the context of the present invention energy absorption and dissipation are the same.
In one embodiment the retractor employs a friction generating device which is set to generate a belt reaction force in the range of about 2-3 kN. This level of energy absorption is employed when a 95th percentile occupant is using the system during frontal crashes at or below a first crash level. A torsion bar is used as a second energy-absorbing or dissipating mechanism to generate a protective force for all occupants during high-speed crashes. The torsion bar reaction torque is set to a range of about 4-6 kN. The default mode of operation of the invention is to use the torsion bar.
As will be seen one of the advantages of the invention is that even if the retractor were to start off in its low energy mode of operation, it is designed to automatically revert to the high level of energy absorption after the seat belt has been extended, for example during a crash. The invention also includes a means for disabling the energy absorbing feature of the torsion bar when an occupant of a slight size is using the seat belt system and more particularly disabling the locking pawl associated with the torsion bar as a means for entering the friction mode of operation.
By way of background, a classic seat belt retractor only offers a modest degree of energy absorption, which occurs as the relatively stiff seat belt stretches as it is loaded by the occupant during an emergency.
It is an object of the present invention to provide to provide a multi-level energy-absorbing seat belt retractor.
The present invention advantageously utilizes the combination of the characteristics of the torsion bar and a friction generating mechanism to provide a multi-level seat belt load-limiting system.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.